Tightening devices of this type usually comprises an epicyclic train coupled to a drive assembly and having a planet gear support frame and an inner gear. An inner socket and an outer socket are connected to the frame and the gear, respectively, such that a nut is tightly screwed on a bolt by the reaction of rotation acting on the two sockets.
However, bolts and nuts of specific size only can be tightened up by one device, so that in order to handle bolts and nuts of various sizes, it is necessary to prepare a plurality of tightening devices which are different in socket size or to disassemble the socket attaching portion for replacement.